Process for preparing ethanebis(methylphosphinic) acid

ABSTRACT

The invention relates to a process for preparing ethanebis(methylphosphinic) acid which comprises reacting ethyne (acetylene) with methyl phophonous acid.  
     The invention further relates to the use of the ethanebis(methylphosphinic) acid prepared by this process for preparing flame retardants or preparing other phosphorus-containing products.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a process for preparingethanebis(methylphosphinic) acid from methylphosphonous acid, and alsoto the use of the products prepared by this process.

[0002] Phosphinic acids and salts of these may be prepared by a varietyof methods, and have been described widely within the literature.

[0003] Organic phosphinic acids, and their salts and esters are knownflame retardants. For example, EP 0 699 708 A1 describes flame-retardantpolyester molding compositions, these being rendered flame-retardant byadding the calcium or aluminum salts of phosphinic or diphosphinicacids. The abovementioned salts are obtained by reacting thecorresponding phosphonic acids with calcium hydroxide or aluminumhydroxide.

[0004] Due to their high phosphorus content and especially theirbidentate nature, the diphosphinic acids are described as highlyeffective reactive flame retardants for polyesters, e.g. for textileapplications. This particularly applies to ethanebis(methylphosphinic)acid, specifically in the form of its glycol ester (DE 22 36 037 A1).

[0005] The preparation of ethane(methylphosphinic) acid is technicallyvery complicated, an example of the procedure being an Arbuzov reactionof diisopropyl methylphosphonous diesters with ethylene bromide [P.Mastalerz, Rocziniki Chem 38 (1964), pp. 61-64], followed by estercleavage. The phosphonous diester used is prepared frommethylphosphonous dichloride by reaction with alcohols.

[0006] Another way of preparing ethanediphosphonic acids is proposed inDE 23 02 523 A1 by reacting alkylphosphonous esters with ethyne(acetylene) and then cleaving the diester with HCl, with formation ofalkyl chlorides. Here again, the alkylphosphonous esters used areprepared from the corresponding phosphonous dihalides by hydrolysis andreaction with alcohols.

[0007] The abovementioned reactions are very difficult to carry out,since their final step is cleavage of the corresponding esters, and thisgives technical difficulties.

[0008] In addition, there are byproducts formed which, like some of theabovementioned starting materials, are toxic, or ignite spontaneously,and/or are corrosive, i.e. are highly undesirable.

SUMMARY OF THE INVENTION

[0009] The object on which the invention is based is therefore toprovide a process which can prepare ethanebis(methylphosphinic) acid andis particularly easy and economic to carry out and gives high yield of asingle product, and in particular dispenses with the complicatedcleavage of diphosphinic esters. This process should also be clearlysuperior to the known processes in its effect on the environment.

[0010] This object is achieved by way of a process for preparingethanebis(methylphosphinic) acids which comprises reacting ethyne(acetylene) with methylphosphonous acid:

[0011] Compared with the processes known hitherto, the process of theinvention has considerable advantages since it produces no halogenatedorganic byproducts, involves no complicated cleavage of phosphinicesters, and also has a positive balance in relation to productdistribution. The process is highly effective and economic to carry out.

DETAILED DESCRIPTION OF THE INVENTION

[0012] Methylphosphonous acid is prepared in a simple and known mannerby hydrolyzing methylphosphonous dichloride.

[0013] The methylphosphonous acid is preferably reacted withethyne(acetylene) in the presence of a free-radical initiator.

[0014] The free-radical initiators used preferably comprise azocompounds.

[0015] The azo compounds are preferably cationic and/or non-cationic azocompounds.

[0016] The cationic azo compounds used preferably comprise2,2′-azobis(2-amidinopropane) dihydrochloride or2,2′-azobis(N,N′-dimethyleneisobutyramidine) dihydrochloride.

[0017] The non-cationic azo compounds used preferably compriseazobis(isobutyronitrile), 4,4′-azobis(4-cyanopentanoic acid), or2,2′-azobis(2-methylbutyronitrile).

[0018] The free-radical initiators used preferably comprise peroxidicinorganic and/or peroxidic organic free-radical initiators.

[0019] The peroxidic inorganic free-radical initiators used preferablycomprise hydrogen peroxide, ammonium peroxodisulfate, and/or potassiumperoxodisulfate.

[0020] The peroxidic organic free-radical initiators used preferablycomprise dibenzoyl peroxide, di-tert-butyl peroxide, and/or peraceticacid.

[0021] A wide selection of suitable free-radical initiators can be foundby way of example in Houben-Weyl, Supplementary volume 20, in thechapter “Polymerisation durch radikalische Initiierung”[Free-radical-initiated polymerization] on pages 15-74.

[0022] The free-radical initiators are preferably metered incontinuously during the reaction.

[0023] The free-radical initiators metered in continuously during thereaction are preferably in the form of a solution in theethyne(acetylene).

[0024] The free-radical initiators metered in continuously during thereaction are preferably in the form of a solution in the solvent used.

[0025] The reaction preferably takes place at a temperature of from 40to 200° C.

[0026] The reaction particularly preferably takes place at a temperatureof from 70 to 130° C.

[0027] The reaction preferably takes place in the presence of a solvent.

[0028] The reaction preferably takes place in acetic acid as solvent.

[0029] The reaction preferably takes place by introducing gaseous ethyne(acetylene) at atmospheric pressure.

[0030] The reaction preferably takes place at superatmospheric pressure.

[0031] The manner of conducting the process is preferably such thatafter partial conversion the precipitating ethanebis(methylphosphinic)acid is filtered off, and further acetylene is added after replacing themethylphosphonous acid consumed.

[0032] The present invention also provides a process in which puremethylphosphonous acid is reacted with ethyne (acetylene) in thepresence of a cationic or non-cationic free-radical initiator or in thepresence of a peroxidic free-radical initiator, to giveethanebis(methylphosphinic) acid.

[0033] The present invention also provides a process in whichmethylphosphonous acid is reacted with ethyne (acetylene) in thepresence of a cationic or non-cationic free-radical initiator or in thepresence of a peroxidic free-radical initiator in acetic acid, to giveethanebis(methylphosphinic) acid, and this is continuously removed fromthe reaction mixture by a circulating filter system, and themethylphosphonous acid consumed is likewise continuously replaced byfresh acid.

[0034] The desired ethane bis(methylphosphinic) acid is obtained withhigh selectivity and high purity.

[0035] Either the methylphosphonous acid or the ethyne (acetylene) maybe used in excess, since the reaction partners always react in a molarratio of 2 to 1 (methylphosphonous acid to ethyne (acetylene)).

[0036] The invention also provides the use of theethanebis(methylphosphinic) acid prepared by the process of theinvention as starting material for preparing flame retardants forpolymers.

[0037] The invention further provides the use of theethanebis(methylphosphinic) acid prepared by the process of theinvention as starting material for preparing flame retardants forthermoplastic polymers, such as polyethylene terephthalate, polybutyleneterephthalate, or polyamide.

[0038] The invention also provides the use of theethanebis(methylphosphinic) acid prepared by the process of theinvention as starting material for preparing flame retardants forthermoset resins, such as unsaturated polyester resins, epoxy resins,polyurethanes, or acrylates.

[0039] Finally, the invention also provides the use of theethanebis(methylphosphinic) acid prepared by the process of theinvention as precursor for the chemical synthesis of otherphosphorus-containing compounds.

EXAMPLES

[0040] The examples below illustrate the invention:

Example 1 Ethanebis(methylphosphinic) Acid

[0041] A solution of 93 g of methanephosphonous acid in 200 g of glacialacetic acid is heated to about 90° C. in a 1 l 5-necked flask equippedwith gas inlet frit, thermometer, stirrer, reflux condenser, andinitiator metering. A solution of 14 g (5 mol %) of ammoniumperoxodisulfate in 30 g of water is metered in over a period of 5 h,with vigorous stirring. At the same time, about 10 l/h of ethyne(acetylene) are conducted through the solution by way of the gas inletfrit, excess acetylene being passed to a flare. The reaction temperaturehere is held at from about 95 to 105° C. Once the acetylene had beenremoved by flushing with nitrogen, the mixture was cooled, whereuponethanebis(methylphosphinic) acid precipitates. This is filtered off,washed twice, each time with 50 ml of acetic acid, and dried at 100° C.under the vacuum provided by a water jet. This gives about 78 g ofethanebis(methyl-phosphinic) acid in the form of colorless crystals witha melting point of 197° C. (70% of theory, based on themethylphosphonous acid used). The mother liquor comprises further finalproduct (ethanebis(methylphosphinic) acid) and may be utilized forfurther reactions.

[0042] Elemental analysis: P: calc. 33.3%, found 33.0%; ³¹P NMR spectrum(D₂O): δ=55 ppm (singlet); purity (³¹P NMR): 99%.

Example 2 Ethanebis(methylphosphinic) Acid

[0043] A solution of 160 g of methylphosphonous acid into 200 g ofglacial acetic acid is heated to about 90° C. in a 1l 5-neck flaskequipped with gas inlet frit, thermometer, stirrer, reflux condenser andinitiator metering. A solution of 19 g (5 mol %) of2,2′-azobis(2-methylbutyronitrile) in 100 g of glacial acetic acid ismetered in over a period of 6 h, with vigorous stirring. At the sametime, about 15 l/h ethyne(acetylene) are conducted through the solutionby way of the gas inlet frit, excess acetylene being passed to a flare.The reaction temperature here is held at from about 95-105° C. Once theacetylene had been removed by flushing with nitrogen, the mixture iscooled, whereupon ethanebis(methylphosphinic) acid precipitates. This isfiltered off, washed twice, each time with 50 ml of acetic acid, anddried out 100° C. under the vacuum provided by a water jet. This givesabout 136 g of ethanebis(methylphosphinic) acid in the form of colorlesscrystals with a melting point of 199° C. and purity (31P-NMR) of 99%(73% of theory, based on the amount of methylphosphonous acid used). Themother liquor comprises further final product(ethanebis(methylphosphinic) acid) and may be utilized for furtherreactions.

1. A process for preparing ethanebis(methylphosphinic) acid, whichcomprises reacting ethyne (acetylene) with methyphophonous acid.
 2. Theprocess as claimed in claim 1 wherein a free-radical initiator ispresent during the reaction.
 3. The process as claimed in claim 1,wherein the free-radical initiators used comprise azo compounds.
 4. Theprocess as claimed in claim 1, wherein the azo compounds are cationicand/or non-cationic azo compounds.
 5. The process as claimed in claim 1,wherein the cationic azo compounds used comprise2,2′-azobis(2-amidinopropane) dihydrochloride or2,2′-azobis(N,N′-dimethyleneisobutyramidine) dihydrochloride, and thenon-cationic azo compounds used comprise azobis(isobutyronitrile),4,4′-azobis(4-cyanopentanoic acid) and/or2,2′-azobis(2-methylbutyronitrile).
 6. The process as claimed in claim1, wherein the free-radical initiators used comprise peroxidic inorganicand/or peroxidic organic free-radical initiators.
 7. The process asclaimed in claim 6, wherein the peroxidic inorganic free-radicalinitiators used comprise hydrogen peroxide, ammonium peroxodisulfate,and/or potassium peroxodisulfate, and the peroxidic organic free-radicalinitiators used comprise dibenzoyl peroxide, di-tert-butyl peroxide,and/or peracetic acid.
 8. The process as claimed in claim 1, wherein thefree-radical initiators are metered in continuously during the reaction.9. The process as claimed in claim 1, wherein the free-radicalinitiators metered in continuously during the reaction are in the formof a solution in the ethyne (acetylene).
 10. The process as claimed inclaim 1, wherein the free-radical initiators metered in continuouslyduring the reaction are in the form of a solution in the solvent used.11. The process as claimed in claim 1, wherein the reaction takes placeat a temperature of from 40 to 200° C., preferably from 70 to 130° C.12. The process as claimed in claim 1, wherein the reaction takes placein the presence of a solvent.
 13. The process as claimed in claim 1,wherein the reaction takes place by introducing gaseous ethyne(acetylene) at atmospheric pressure.
 14. The process as claimed in claim1, wherein the reaction takes place at superatmospheric pressure. 15.The process as claimed in claim 1, wherein methylphosphonous acid isreacted with ethyne (acetylene) in the presence of a cationic ornon-cationic free-radical initiator or in the presence of a peroxidicfree-radical initiator, to give ethanebis(methylphosphinic) acid. 16.The process as claimed in claim 1, wherein methanephosphonous acid isreacted with ethyne (acetylene) in the presence of a cationic ornon-cationic free-radical initiator or in the presence of a peroxidicfree-radical initiator in acetic acid, to giveethanebis(methylalkylphosphinic) acid, and this is continuously removedfrom the reaction mixture by a circulating filter system, and themethylphosphonous acid consumed is likewise continuously replaced byfresh acid.
 17. The use of the ethanebis(methylalkylphosphinic) acidprepared as claimed in claim 1 as starting material for preparing flameretardants for polymers.
 18. The use as claimed in claim 17, wherein thepolymers are thermoplastic polymers, such as polyethylene terephthalate,polybutylene terephthalate, or polyamide.
 19. The use as claimed inclaim 17, wherein the polymers are thermoset resins, such as unsaturatedpolyester resins, epoxy resins, polyurethanes, or acrylates.
 20. The useof the ethanebis(methylalkylphosphinic) acid prepared as claimed inclaim 1 as a precursor for the chemical synthesis of otherphosphorus-containing compounds.